Manufacture of paper



Patented Dem-26, 1939 MANUFACTURE OF PAPER Warner J. Merrill, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilming'ton, Del., a corporation of Delaware No Drawing.

Application July 6, 1936,

Serial No. 89,254

4 Claims.

This invention relates to the preparation of grease-proof paper.

This application is a continuation-in-part of my application No. 731,602, now Patent No. 2,047,-

In the paper making art there are two general methods of imparting grease resistance tc paper. In one of these methods grease resistance is obtained by forming a sheet from special pulp which has been beaten exhaustively to a very slow, gelatinous state. This mechanical method is costly because a special pulp is required, power consumption is high, and the conversion rate is low in comparison with grades of paper made from freer pulps. Glassine paper is an example of greaseproof paper made from highly beaten pulp. Various chemical methods are also known. In general, these consist in applying to the formed sheet strong parchmentizing reagents such as sulfuric acid, mercerizing caustic, zinc chloride, etc. These chemical methods are also costly because of the concentrated reagent required. They have the additional disadvantage that to obtain uniform results very close control is involved.

. In the paper industry various size materials are incorporated bytreatment of the furnish before sheet formation or by surface sizing sheet at an intermediate stage in the drying operation or during the caleridering operation. I find, however, that neither beater sizing nor surface sizing, using known systems of application, is capable of efiecting a high degree of grease-proofness when the size is applied to free paper stock and when only non-parchmentizing reagents are used.

By the novel process described in this application, practically any desired degree of greaseproofness may be imparted to paper using paper pulp of any desired grade and using only comparatively dilute reagents under conditions which do not involve parchmentizing the cellulose fiber.

This invention has as an object a process capable of making paper having various degrees of grease resistance combined with high wet strength. Another object is to provide a process for producing this type of paper in a greater range of weights and grades than is possible by the present known processes of producing greaseproof paper. A still further object is to provide a process of preparing highly grease-proof paper by applying sizes to paper made from a free paper stock.

These objects are accomplished by applying to the paper a new combination of beater size and surface size wherein size materials of a specified class are used and wherein the surface size contains a solvent which is adapted to act as a swelling agent for the precipitated beater size but does not parchmentize the cellulose fiber.

Film-forming materials for use in the beater size solution are selected from those hydrophilic colloids which are soluble in aqueous medium to.

form a viscous solutionv capable, on precipitation, of laying down a continuous film which is substantially free from regenerated cellulose. The filmformer is further characterized by the fact that the viscosity of its solution in aqueous medium is not less than two poises when measured at 5% concentration and at C.

The hydrophilic colloid used in the surface size may be the same as that used in the beater size, or it may be difierent. the surface size solution carry, in addition to a suitable film-former having the viscosity characteristics specified for the film-former of the beater size, a solvent which is capable under conditions of use of acting as a swelling agent for the precipitated beater size, but is not adapted under such conditions to parchmentize the cellulose fiber.

The use in the beater size of materials, such as viscose, which on precipitation yield regenerated cellulose, is excluded because such materials, after precipitation and drying, are swollen only with difficulty and bypthe use of concentrated reagents which parchmentize the cellulose fiber.

It is essential, however, that The hydrophilic colloids referred to above form viscous solutions in the low concentration of the aqueous systems. They are solubilized in the aqueous medium by an acid or base and are precipitated from solution by partial 'or complete removal of the acid or base or by the addition of certain metal ions. Colloids answering the foregoing' requirements are well known to those skilled in the paper making art and include among other materials substances such as deacetylated chitin, sodium cellulose glycolate, methyl cellulose, glycol cellulose, etc. A colloid particularly suitable for the purpose of the present invention is deacetylated chitin such as that disclosed in United States Patent 2,040,879 and which may be prepared by treating lobster or crab shells, which have been freed from extraneous matter with 40% aqueous caustic soda and at 110 C. until the product is soluble in dilute acetic acid, after which the product'is washed to neutrality. For use in a size solution, the deacetylated chitin is dissolved in dilute acid, for, example, dilute acetic acid. v

A more complete description of the preparation of the deacetylated chitin is given in United States Patent 2,040,879, also in the mentioned Patent 2,047,218, which latter also describes the production of grease-proof paper with solutions of deacetylated chitin as claimed herein. Thus, the deacetylated chitin is applied in such solution for the production of grease-proof paper in accordance with the first example below (Example IV of the mentioned application) in which the procedure outlined above is used.

Example I A bleached sulfite pulp was beaten until the fibers were shredded and hydrated into what is known as a slow stock, that is, stock of the grade commonly used for bond paper. To 'a beater containing a water slurry of the pulp a quantity of a solution of the acetate of partially deacetylated chitin (3% partially deacetylated chitincontent) was added which gave a partially deacetylated chitin content of 2% based on the dry weight of fiber. After several minutes of agitation in the beater dilute ammonia was slowly added with further agitation until the sized slurry gave an alkaline reaction with litmus paper. The sized pulp was formed into wet sheets, part of the excess water was removed by draining and the partially dried sheets were hot calendered. until substantially dry. The sheets were then tub sized in 0.45% acetic aqueous solution of partially deacetylated chitin (which gave a total partially deacetylated chitin content of 4.5% based on the dry fiber content), partially dried and then hot calendered until dry.

The sheets, in addition to having unusual water resistance and high wet and dry strength, showed excellent resistance to the penetration of oil and grease.

Other examples illustrating the practice of my invention with deacetylated chitin are given below:

Example 11 A slurry of wood pulp (8889% alpha cellulose) beaten to freeness of book paper stock is treated with a 3% solution of deacetylated chitin dissolved in dilute acetic acid in the beater before the pulp is converted into paper; The process is somewhat similar to that of beater sizing pulp with rosin size. After the deacetylated chitin is thoroughly mixed with the paper furnish the slurry is made alkaline by the addition of ammonia which precipitates the deacetylated chitin onto the paper furnish. This treated pulp is then converted into paper by passing over a Fourdrinier machine. The deacetylated chitin added to the paper furnish in the beater amounts to 2% by weight of the dry paper. After the paper is dry, both surfaces are treated further with the same solution by passing sheets through a solution of deacetylated chitin in dilute acetic acid, passing through squeeze rolls and then over cylinders of a paper drying machine. This product has properties different than the ordinary rosin sized paper of the same weight and density in that its grease resistance is about twenty times as high as that of a rosin sized sheet, this result being due to the swelling action of the surface size solution on the precipitated beater size.

Example III This example illustrates the production of a so-called greaseproof paper which is formed by treating a highly fibrillated pulp with deacetylated chitin before sheet formation and after the sheet has been dried. It is different from that obtained in Example II in that the, pulp has been beaten to a greater degree which gives greater grease-proofness per unit amount of on dry furnish weight) mentioned alkali-soluble scribed herein). A portion of this pulp was sized in a one-half lb. capacity beater as follows:

Grams 3.9% consistency pulp at 77 seconds free- The solution of deacetylated chitin 2% based was, mxed thoroughly with the pulp before addition of the ammonium hydroxide solution, which precipitated the deacetylated chitin onto the fibers. The pH of the sized pulp was 9.3. The freeness of the sized pulp was 74 seconds. Sheets were formed by regular procedure on a sheet mold, couched from the forming wire, placed between felts and pressed to 40% dryness in a flat bed press. The sheets were then placed on a drier and dried to 88% bone dry. At this point they were dipped into a solution of deacetylated chitin in dilute acetic acid and passed through squeeze rolls leaving sufficient amount of the solution on the surfaces to add 1% of deacetylated chitin based on the dry sheet weight. The sheets were then dried by passing over a drying cylinder. Control sheets were made from the same ingredients except that the amount of deacetylated chitin solution added to the furnish was increased to 374 grams in order to obtain 3% film-former based on dry furnish. No surface treatment was given these control sheets. The turpentine resistance of these sheets was: for those merely given the beater treatment, 24 minutes; for those with combined beater and surface sizing treatment, 258 minutes. The basis weight of the sheets was 43 lb. for a ream of 25x38x500.

The values given for the turpentine resistance represent theaverage value taken from 16 tests measuring the time required for turpentine to pass through the weakest spot of a 0.3 sq. in. test area.

Although the water drained from this pulp nine times faster than from a pulp of the freeness required to make grease-proof paper of the glassine type, the grease resistance of the finished sheet was greater than that of the sheet made from pulp beaten to what is ordinarily considered grease-proof freeness. This permits higher machine speed.

These results show the great advantage of applying the surface size over the beater size, under conditions such that the precipitated beater size is swollen by the acetic acid present in the surface size. A complete description of the construction of the tester used for measuring pulp freeness is given in United States Patent 1,857,100. The procedure used for measuring the pulp freeness on this tester follows that method except that the amount of stock has been reduced to 0.5 g. bone dry weight of fiber for one liter of water.

Instead of deacetylated chitin I may use as the film-former in the size solutions other hydrophilic colloids having the properties already defined. For example, I may use cellulose derivatives which are soluble in aqueous medium, and which, on precipitation therefrom, are capable of being reswollen by treatment with aqueous reagents which, under conditions of use, are not capable of parchmentizing the cellulose fiber. As examples of such film-formers, there may be celulose ethers such as was 18 minutes, that of I 2,184,812 li-soluble methyl cellulose, glycol cellulose,

etc.

Example IV eple illustrates the production of a grease resistant paper by beater sizing with 2% methyl cellulose and, after drying the sheet, surface sizing with an additional 1% of the same film-former. in both size solutions the filmformer is dissolved in dilute alkali.

The beaten pulp described under Example III was treated as follows:

, Grams 3.9% .consistency pulp at 77 seconds freeness 4000 Water 3000 1% solution of low substituted methyl cellulose in 6% sodium hydroxide so1ution of sulfuric acid 450 The pH of the sized pulp was 5.1, and its freeness was 9'? seconds.

The ingredients were added in the order named allowing ample time for complete mixture of the low substituted methyl cellulose solution with the pulp furnish before the solution of sulfuric acid was added to cause precepitation. The sheets (43 lb. basis weight for a x 38 x 500 ream) were formed on a regular sheet mold and they were dried to 90% bone dry. These dried sheets were then surface sized with an additional 1% of filmformer by dipping into a solution of low substituted methyl cellulose in 6% sodium hydroxide, passing through squeeze rolls to remove excess solution, dipping in dilute sulfuric acid solution to precipitate the low substituted methyl cellulose,

washing in water to remove sulfuric acid neutralizing with dilute ammonia and drying over the regular drying cylinder to 92% bone dry. Control sheets were made from the same ingredients except that the amount of methyl cellulose solution was increased to 468 g. and the sulfuric acid solution to 675 g. to obtain 3% film-former in the sheet. These sheets were dried to 92% bone dry without giving any surface treatment. As further control sheets from the same furnish were merely surface sized with enough methyl cellulose solution to obtain 3% size solids the average grease resistance of the beater sized sheets the surface sized sheets was 22 minutes, and that of and surface sized sheet was 220 minutes.

This enormous increase in grease resistance illustrates the effect of selecting the surface size solution to effect swelling action in the precipitated methyl cellulose. The surface size does not, however, to parchmentize the cellulose fiber.

The surface size may be identical with the beater sizeor it may be different, but in either case it is necessary that the surface size contai'n a swelling agent for the precipitated beater size. Thus, the surface size solution may be an alkalisoluble methyl cellulose or glycol cellulose or it may be some other hydrophilic colloid having the mentioned viscosity characteristics provided a solvent or swelling agent is present which is adapted under the conditions of use to effect a swelling actionon the precipitated methyl cellulose.

The hydrophilic colloids which are most effectively 'used in'the practice of this inventioniare deacetylated chitin, low substituted methyl and glycol celluloses (soluble in 8 7% sodium hydroxide on cooling), cellulose glycolic acid, alkali under the conditions present act soluble, cellulom glycolic acid whose onium or sodium salt is soluble in water.

- t However, the process of the invention shows to the greatest advantage whenit is applied to free, lightly beaten paper pulp, which consumes comparatively little power in the beating operation and which in sheet form is so quick draining that the paper machine may be operated at maximum speed. As usually prepared, glassine paper,

- although it may be made fairly grease-proof, in-

volves high power consumption in the long beating of the pulp. Furthermore, the sheet is extremely slow draining so that the machine must be operated at a,proportionately low speed.

The more impervious products of this invention are particularly useful for packaging foodstuffs and they may be used wherever high grease resistance of paper is required. The more porous type are useful wherever control of printing ink penetration is required.

The grease-proof papers of the present invention, also those prepared by the process disclosed in the mentioned Patent 2,047,218, are usefully employed in the manufacture of wrappers for dynamite cartridges, and especially for the purpose of cutting down the rate of vapor diffusion in the case where the cartridge is intended to contain powder having a volatile explosive constituent, for example, ethylene glycol dinitrate.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof,'it is to be understod that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

I claim:'

1. A process of preparing grease-proof paper which comprises mixing paper pulp before sheet formation with size comprising solution in aqueous medium of a film-former consisting of a hydrophilic colloid capable on precipitation from dilute solution of laying down a continuous film,

said colloid having the property of forming an aqueous solution of viscosity not less than two poises when tested at 5% concentration and 25 C., precipitating said hydrophilic colloid on the paper pulp, forming the pulp into a sheet, drying and then swelling the precipitated hydrophilic colloid by surface sizing the paper with aqueous thecombinedbeater size solution which does not parchmentize the sheet-fiber and which contains a film-former selected from the class of hydrophilic colloids having the mentioned viscosity characteristics, the hydrophilic colloid in both of said size solutions being readily swollen by reagents which do not parchmentize cellulose fiber, and the hydrophilic colloid in one of said size solutions being 'deacetylated chitin.

2. The process set forth in claim 1 in which said hydrophilic colloid in said solution mixed with the paper pulp before sheet formation is deacetylated chitin.

'3. The process set forth in claim 1 in which said hydrophilic colloid in said solution. applied to the dried sheet is deacetylated chitin.

4. A process of preparing grease-proof paper which comprises mixing paper pulp before sheet formation with size comprising solution in aqueone medium of deacetylated chitin, precipitating the deacetylated chitin on the paper pulp, forming the pulp into a sheet, drying and then swelling the-precipitated deacetylated chitin by surface sizing the paper with size solution in aqueous medium of, deacetylated chitin.

WARNER JJIERRILL ll 

